With rain recorded on fifteen days since August 1stat ADAS Boxworth in Cambridgeshire growers are being advised to look out for phoma in OSR crops. Thats thewarning from ADAS plant pathologist Dr Faye Ritchie.
Many areas have experienced a wet end to summer andthere is a real possibility that thresholds could be reached a little earlierthis season. The trigger for air-borne spore release is twenty rain days fromAugust 1st. We only need a few more rain events and well be there.
And with some growers delaying drilling to counterCSFB it could heighten the risk further. The smaller the plant the easier itis for the disease to spread from leaf to petiole and then stems. If thesecrops havent achieved sufficient growth by the time we see Phoma leaf spot they could beextremely vulnerable, she cautions.
But the threat isnt just phoma, light leaf spot (LLS) could also threaten too. LLS wasagain very evident last season the Defra oilseed rape disease survey datashows 85% of crops were affected in England and Wales with 34% plants showingsymptoms in affected crops last spring. Stubbles from the previous years cropact as a reservoir and this wet spell will have done little to check productionof air-borne spores.
With both phomaand LLS to consider it is one reason why Dr Ritchie favours split autumn foliartreatments. A first application when 10 to 20% plants are affected by phoma leaf spot and a secondapplication approximately four to six weeks following a first treatment,assuming thresholds are reached, should be considered, she notes.
Bayer CropScience technical manager Tim Nicholson isalso worried about the vulnerability of late drilled crops. He points out thatat the start of the month 40% of the wheat crop was still to be harvested inmany areas. Although some OSR has gone into the ground the weather hasintervened and combines have been stuck in the shed. With growers forced intodrilling later large parts of the OSR area could be at high risk to phoma.
And he also has concerns with LLS. Withblack-grass driving rotational strategies for many growers, some will not wantto plough up non-dormant seed. The result is that there could be plenty oftrash around driving ascospore production. If it stays wet leaf wetness is thenall-important for germination of ascospores and subsequently secondaryinfection via conidia, so both diseases could threaten this autumn, he suggests.
For him it reinforces the arguments for splitapplications. If phomathresholds are reached early then there is a good chance you will have to getback in anyway. The second spray is the opportunity to hit LLS too.
But even with the possibility of earlier infectionhe says growers dont need to move away from current split sprayrecommendations. Earlier applications could leave new growth exposed to LLS ifthe second spray isnt effective against the disease. Its why we advise a 0.32l/ha dose of Proline275 (prothioconazole) at the first treatmentfollowed by a 0.46 l/ha dose for the second. The gap to pre-stem extension canbe several weeks and you really do need the potency of a robust dose of aneffective active, he concludes.